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POB 2 > Lecture 6 Virus > Flashcards

Lecture 6 Virus Flashcards

1
Q

Key concepts

A

➢ A virus consists of a nucleic acid
surrounded by a protein coat
➢ Viruses replicate only in host cells
➢ Viruses and prions are formidable
pathogens in animals and plants

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2
Q

A Glimpse of History in tobacco

A

▪ Tobacco mosaic disease (1890s)
* D. M. Iwanowsky, Martinus Beijerinck determined caused
by “filterable virus” too small to be seen with light
microscope, passed through filters for bacteria
* F. W. Twort and F. d’Herelle discovered “filterable virus”
that destroyed bacteria
* Virus means “poison”

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3
Q

Viruses: Obligate Intracellular Parasites

A

▪ Genetic information: DNA or RNA contained within
protein protective coat
* Inert particles: no metabolism, replication, motility
* Genome hijacks host cell’s replication machinery
* Inert outside cells; inside, direct activities of cell
* Infectious agents, not organisms
* Can classify generally based on type of cell they infect:
eukaryotic or prokaryotic

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4
Q
  1. General Characteristics of Viruses
A

Virion (viral particle) is nucleic acid, protein coat
* Protein coat is capsid: protects nucleic acids
○ Carries required enzymes
○ Composed of identical subunits called capsomeres
* Nucleocapsid = NA + capsid
* Matrix protein between nucleocapsid and envelope
* Enveloped viruses have lipid bilayer envelope
* Non-enveloped (naked) viruses lack envelope; more resistant to disinfectants

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5
Q

virus size/ shape

A

slide 6

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6
Q

Viral Structure

A

▪ Viral genome is either DNA or RNA, never both
* Useful for classification (that is, DNA or RNA viruses)
* Genome linear or circular
* Double- or single-stranded
▪ Viruses have protein components for attachment
* Phages have tail fibers
* Many animal viruses have spikes
* Allow virion to attach to specific receptor sites

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7
Q

Viral Shape

A

▪ Generally four different shapes
* Icosahedral (Adenovirus)
* Helical or rod (TMV)
* Complex (bacteriophages like T2)
* Spherical (influenza, coronavirus)

slide 8

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8
Q
  1. Replicative cycle
A

Generally four steps
* Attachment and Entry
* Synthesis
* Assembly of particles
* Exit of the cell

slide 9

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9
Q

Replication of Bacteriophages and the types of relationships

A

▪ Viruses that infect bacteria
▪ 3 general types of bacteriophages based on relationship
with host
* Lytic phages
* Temperate phages
* Filamentous phages

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10
Q

Lytic Phage Infections

A

● Lytic or virulent phages exit host
● Cell is lysed
○ Productive infection: new particles formed
● T4 phage (dsDNA) as model; entire process ~30 min
● Five step process
○ Attachment
○ Genome entry
○ Synthesis
○ Assembly
○ Release

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11
Q

Lytic Cycle

A

slide 12

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12
Q

Lytic Cycle (time lapse)

A

slide 13

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13
Q

Lysogenic Cycle

A

slide 14

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14
Q

Bacterial Defenses Against Phages

A
  1. Preventing Phage Attachment (NO RECEPTORS)
  2. Restriction enzymes (restrict a phage’s ability to replicate
    within the bacterium
  3. CRISPR-Cas system
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15
Q
  1. Preventing Phage Attachment
A

Alter or cover specific receptors on cell surface
* May have other benefits to bacteria
* S. aureus produces protein A, which masks phage receptors;
also protects against certain human host defenses
* Capsules, slime layers (biofilms) also mask receptor

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16
Q
  1. Restriction-Modification Systems: two enzymes
A

Restriction enzymes (RE)
● Restrict a phage’s ability to replicate within the bacterium
● Recognize, cut short nucleotide sequences (phage DNA)
● Bacteria have different versions; hundreds of varieties

slide 19

17
Q
  1. Restriction-Modification Systems: two enzymes
A

Methylase
Methylate host sequences normally recognized by RE
* Restriction enzymes do not recognize bacterial DNA
* Enzymes methylate (CH3) bacterial DNA

slide 20

18
Q

3.CRISPR-Cas system

A
  • Bacterial immune system
  • CRISPR - clustered regularly interspaced short palindromic repeats
  • Cas (CRISPR-associated protein) degrades RNA

slide 21

19
Q

Replication of Animal viruses

A

Key Characteristics of animal viruses
● Genome structure
- Nucleic acid (RNA or DNA)
- Strandedness: single-strand(ss) or double-strand (ds)
● Enveloped vs non enveloped
● Hosts infected
● Other characteristics
- viral shape
- disease symptoms

20
Q

Central dogma of Biology

A

slide 23

think of what should the host provide and what should the virus provide

21
Q

Baltimore classification System of Viruses

A

slide 24-26

+ strand used as a template

22
Q

Synthesis of viral genetic material

A

● Expression of viral genes to produce viral structural and
catalytic genes (for example, capsid proteins, enzymes
required for replication)
● Synthesis of multiple copies of genome
● Most DNA viruses multiply in nucleus
● Enter through nuclear pores following penetration
● Three general replication strategies depending on type of
genome of virus
○ DNA viruses ( Herpes, Pox, HPV)
○ RNA viruses (Flu, Covid)
○ Reverse transcribing viruses (Retroviruses)-HIV

23
Q

Sense vs nonsense review

A

slide 28

24
Q

What is Antisense RNA

A

Antisense RNA is a single-stranded RNA that is complementary to the messenger RNA (mRNA) strand transcribed within a cell

they are introduced in a cell to inhibit the translation machinery by base pairing with the sense RNA and activating the RNase H, to develop a particular novel transgenic

mRNA = Sense

slide 29

25
Q

Different polymerases

A

DNA-dependent DNA polymerases
- eg. prokaryotic and eukaryotic DNA polymersases

DNA-dependent RNA polymerase
- eg. RNA polymerases and DNA primase’

RNA-dependent DNA polymerase
-eg. reverse transcriptase

RNA-dependent RNA polymerases
-eg. RNA replicase

26
Q

Replication of DNA viruses

A

Usually in nucleus
* dsDNA replication straightforward
* ssDNA similar except complement first synthesized

slide 31

27
Q

Replication of RNA viruses

A
  • Majority single-stranded; replicate in cytoplasm
  • Require virally encoded RNA polymerase (replicase), which lacks
    proofreading, allows antigenic drift (influenza viruses)
  • ss (+) RNA used as mRNA
  • ss (–) RNA, dsRNA viruses
    carry replicase to synthesize (+) strand
  • Some RNA viruses segmented;
    Reassortment results in antigenic shift
    Polymerases used
    RNA polymerase-RNA dependent = Replicase

slide 32

28
Q

Replicative cycle of an enveloped RNA virus.

A

slide 33

29
Q

Replication of reverse-transcribing viruses

A
  • Encode reverse transcriptase: makes DNA from RNA
  • Retroviruses have ss (+) RNA genome (HIV)
  • Reverse transcriptase synthesizes single DNA strand
  • Complementary strand synthesized; dsDNA integrated into host cell
    chromosome
  • Can direct productive infection or remain latent
  • Cannot be eliminated
    Polymerases used
    DNA polymerase –RNA dependent =
    reverse transcriptase
    RNA polymerase-DNA dependent
    DNA polymerase –DNA dependent
30
Q

The replicative cycle of HIV

A

3 genes in common: gag, pol, and env
* gag gene - viral protein coat
* pol gene - reverse transcriptase and
integrase (inserts the viral DNA into a host
chromosome)
* env gene - glycoproteins that appear
on the surface of the viral envelope
* reverse transcriptase is very error
prone – high mutation rate (evolve
rapidly)

31
Q

Human Immunodeficiency Virus (HIV)

A
  • AIDS caused by HIV-1 and HIV-2
  • HIV-1 evolved from a similar virus SIV
    cpz
    simian
    immunodeficiency virus found in chimpanzees
    and was transmitted to humans.
  • SIV
    cpz
    arose from recombination between
    retroviruses from red-capped mangabeys and
    from greater spot-nosed monkeys
  • Independent transfers of SIVcpz to humans gave
    rise to different groups (strains) of HIV-1
32
Q

Why do new viruses emerge?

A
  1. Mutation of existing viruses into new viruses
  2. Spread due to affordable international travel
  3. Spread of existing viruses from other animals
33
Q

Influenza

A

● 36,000 death/year
● Structure: 8 sense viral RNA and two surface antigens
● Rapid changes occur through genetic recombination.
● Three main types:
○ Influenza A
○ Influenza B
○ Influenza C
● Cases influenza A: divided into subtypes based upon
expression of hemagglutinin (HA) and
neuraminidase (NA).

slide 37-38

34
Q

Coronaviruses

A
  • Spherical or
    pleomorphic
    enveloped particles
    containing
  • single-stranded
    (positive-sense) RNA
    associated with a
  • nucleoprotein within a
    capsid composed of
    matrix protein.
  • envelope bears
    club-shaped
    glycoprotein
    projection
  • glycoprotein spike on
    the surface, which
    mediates receptor
    binding and cell entry
    during infection

slide 39

35
Q

Prions: Proteins as Infectious Agents

A

▪ Prions are proteinaceous infectious agents
* Composed solely of protein; no nucleic acids
* Linked to slow, fatal human diseases; animal diseases
* Usually transmissible only within species
* Mad cow disease in England
* Creutzfeldt-Jakob disease in humans killed 175 people

slide 40

36
Q

Creutzfeldt-Jakob disease

A

slide 41

37
Q

Spongiform encephalopathies

A

Prion proteins accumulate in neural tissue
* Neurons die
* Tissues develop holes
* Brain function deteriorates
Characteristic appearance
gives rise to general term for all prion diseases: transmissible
spongiform encephalopathies

POB 2 (25 decks)

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